Defying expectations in a region devastated by AIDS, rates of H.I.V. infection in Zimbabwe have fallen precipitously in the last decade.

In 1997, an estimated 29 percent of adults were infected. A decade later, 16 percent were.

The basic explanation is simple, according to a new study published online last week by PLoS Medicine: Zimbabweans had less extramarital sex.

Seeing other people die was the “dominant” reason for that, according to the study, which was based on large demographic surveys and interviews with more than 200 Zimbabweans.

The second biggest factor was the collapse of the economy under President Robert Mugabe, which cost most citizens 90 percent of their purchasing power.

Men said that left them less able to buy sex or pay for multiple girlfriends. “Concurrent relationships” – in which men and women have several sexual partners over the years, but infrequently – are considered a prime driver of the epidemic in southern Africa.

Sexual norms also changed, the study said. Fewer women went to beer halls to meet men, and having a venereal disease became a badge of shame rather than proof of masculinity.

AIDS education campaigns “probably” helped, the authors said. Condom use did not increase during the study period, though it was already close to 60 percent for extramarital sex. Also, Zimbabweans had more education and were more often married than people in nearby countries with high infection rates. They also more often watched relatives die at home.